This invention relates to an actuator driving circuit capable of preventing howling sounds when an actuator is driven, and an actuator driving device provided with such an actuator driving circuit, and an actuator driving method.
Conventionally, there has been known an actuator in which a driving member is fixedly attached to an electromechanical transducing element such as a piezoelectric element, and is reciprocatingly displaced by applying a drive voltage comprising a pulse having a serrated waveform to this electromechanical transducing element to expand and contract it, thereby moving a movable member coupled with the driving member by friction along expanding and contracting directions. Such an actuator is hereinafter referred to as "piezoelectric actuator").
FIG. 14 is a block diagram showing a driving circuit for the piezoelectric actuator. A voltage of about 30V is used to operate the piezoelectric actuator. Accordingly, in the case that a voltage in a power supply unit 100 is 5V as shown in FIG. 14, an output voltage from a dc-to-dc converter 102 having a switching regulator circuit 101 is boosted to 30V.
A pulse signal generated in a pulse signal generating circuit 104 of a CPU 103 has its frequency divided into a specified frequency by a pulse frequency dividing circuit 105, and a drive pulse signal having a serrated waveform is generated from this pulse and the output voltage of 30V in a drive pulse signal generating circuit 106 and applied to a piezoelectric actuator 107.
FIG. 15 is a diagram showing a circuit construction of the switching regulator circuit 101 of FIG. 14, and FIG. 16 is a timing chart showing waveforms of voltages from the respective outputs in FIG. 15.
A field-effect transistor Q is turned on and off by a PWM drive voltage DRIVE from a drive circuit 110. By turning the transistor Q on and off, storage of energy in a coil L and discharge thereof from the coil L are continuously performed, and a capacitor C is charged with this energy to increase an output voltage VOUT as shown in FIG. 16.
On the other hand, a feedback signal FB is obtained by amplifying a difference between the output voltage VOUT and a reference voltage VREF by an error amplifier 111, and a serrated wave signal SAW is outputted from a serrated wave signal generating circuit 112 in accordance with a synchronization pulse SYNC of a specified frequency. A PWM signal is outputted after the feedback signal FB and the serrated wave signal SAW are compared by a PWM comparator 113, and the PWM drive voltage DRIVE from the drive circuit 110 is controlled, thereby controlling an on-off duty ratio of the transistor Q. As a result, the output voltage VOUT is held at a specified level.
In the switching regulator circuit of FIG. 15, power noise is generated when the energy stored in the coil L is discharged by turning the transistor Q off as shown in the waveform of the output voltage VOUT in FIG. 16. This power noise affects the output voltage via wiring, with the result that the output voltage may be rippled and/or spike voltages SP may be added thereto as shown in FIG. 16. If the drive pulse signal having a serrated waveform comprising a driving voltage experiencing this spike voltage SP is applied to the piezoelectric actuator 107, the piezoelectric actuator 107 slightly expands and contracts in synchronism with the spike voltage SP. These expansion and contraction generate sounds.
If a switching frequency obtained by turning the transistor Q on and off and the frequency of the drive pulse signal fed to the piezoelectric actuator 107 are both ultrasonic frequency, the sounds generated by the expansion and the contraction of the piezoelectric actuator 107 by the drive pulse signal and by the spike voltage lie outside an audible range by humans. However, if the expansion and the contraction by the drive pulse signal and those by the spike voltage interfere each other, the so-called "howling" occurs. At this stage, if "howling" of an audible frequency is produced, this becomes a noise, thereby giving an unpleasant feeling to humans.
Japanese Unexamined Patent Publication No. 8-80075 discloses an apparatus provided with a plurality of ultrasonic actuators whose drive frequencies are ultrasonic frequencies, in which the respective drive frequencies are so set as not to cause mechanical howling between the actuators.